1. Field of Endeavor
The present invention relates to inertial classifiers, and more particularly, to a virtual impactor. The virtual impactor system of the present invention will separate a fluid stream into a first particle flow component containing particles essentially greater than a selected size and a second particle flow component containing particles essentially smaller than the selected size.
2. State of Technology
Impactors belong to a class of instruments called inertial classifiers, which separate particles in a fluid stream based on the inertia of particles. In general these instruments operate by forcing a change in the direction of the fluid stream containing the particles. Because of the particles inertia the particles cannot follow the fluid stream. If an obstacle is placed in the path of the fluid stream, the fluid is deflected around the obstacle while the particles are less able to deflect around the obstacle. If the particle inertia exceeds a minimum quantity, the particle cannot deflect around the obstacle and will be caught by the obstacle.
The most common inertial classifiers are cyclones, impactors, and virtual impactors. A cyclone operates by forcing an air stream tangentially into a cylinder causing it to circulate around the cylinder. Particles in the air stream having sufficient inertia will collide with the interior wall. Impactors operate by directing a jet or jets of air against a relatively flat surface perpendicular to the air jet or jets. Particles suspended in the air and with sufficient inertia will impact on the flat surface.
For virtual impactors, the air stream is directed at a relatively stagnant air space that acts as a virtual surface. In this case the particles remain suspended in the air as they impact into a relatively stagnant air or low flow space. This low flow stream provides an enriched concentration of the larger size particles for subsequent treatment or sampling.
U.S. Pat. No. 5,425,802, patented Jun. 20, 1995, to Robert M. Burton, et al, assigned to The United States of American as represented by the Administrator of Environmental Protection Agency and President and Fellows of Harvard, shows a virtual impactor for removing particles from an airstream. The virtual impactor comprises nozzle means for accelerating an entering airstream, particle receiving means positioned downstream from the nozzle means, and a chamber in fluid communication with the gap between the nozzle means and the receiving means. The nozzle means comprises an inlet and an elongated outlet having a width dimension of between about 0.007 and 0.010 inches, and further having a longitudinal axis normal to and passing through the center of the elongated outlet. The particle receiving means comprises an elongated inlet having a width dimension of between about 0.013 and 0.015 inches and an outlet and further has a longitudinal axis normal to and passing through the center of the elongated inlet. The particle receiving means is positioned downstream from the outlet of the nozzle means so that the flow gap therebetween is between about 0.008 and 0.012 inches, and is further positioned so that the longitudinal axis of the nozzle means and the longitudinal axis of the receiving means are substantially coaxial and so that the width dimension of the nozzle means outlet and the width dimension of the receiving means inlet are substantially parallel. The chamber is configured to be in fluid communication with a vacuum source, as is the outlet of the particle receiving means.
U.S. Pat. No. 5,040,424, patented Aug. 20, 1991, to Virgil A. Maple, et al, assigned to Regents of the University of Minnesota, shows a high-volume aerosol sampling inlet housing which provides smooth inlet flow to a 10 micron classification device in a high volume flow. The high volume sampler with which the inlet is used establishes a high flow, for example, 40 cubic feet per minute. The air flow into the inlet has a standard 40 cubic feet per minute leading to the high volume sampler which requires a secondary inlet flow of about two cubic feet per minute needed for particle classification. The two cubic feet per minute flow is exhausted at a separate outlet and is not connected to the standard high volume sampler. Thus, a total flow of 42 cubic feet per minute enters the inlet. The entrance opening to the inlet is an annular opening below a dome cover. Screens are provided to keep any bugs or large debris from entering the inlet housing. The debris-free air flow is passed through the desired impactor device, and the large particles will be collected with the secondary outlet flow of only two cubic feet per minute while the smaller particles are carried out by the major flow of 40 cfm to the high volume sampler filter placed below. The larger particles are thus inertially separated from the major flow and are flushed by the smaller secondary or minor flow. The major flow through to the high volume sampler is maintained at the standard 40 cubic feet per minute. The particles in the inlet air stream are separated into size classifications larger and smaller than 10 microns. The large particles that are flushed out with the two cubic feet per minute flow can either be removed from the air stream by a second filter, or analyzed in a conventional impactor or some other device, or may be allowed to pass through the air pump and be blown back into the atmosphere.
U.S. Pat. No. 5,183,481, patented Feb. 2, 1993, to William Felder, assigned to Aerochem Research Laboratories, Inc., shows a supersonic virtual impactor. A supersonic gas flow is employed with a virtual impactor to separate fine particles completely from the gas. The carrying gas and fine particles are accelerated to supersonic speeds and then impacted against a virtual impactor. When the supersonic stream strikes the virtual impactor, a shock wave forms in the gas stream near the impactor surface. The carrying gas turns sharply away while the particles in the gas stream, carried by their inertia, continue in their original direction and pass into the virtual impactor. On the downstream side of the virtual impactor surface, a non-contaminating inert gas maintains a pressure equal to or greater than the pressure of the carrying gas between the virtual impactor surface and the shock wave.
Hounam and Sherwood described the use of two stages of multi-jet virtual iimpactors using round jets for generating relatively monodisperse particles from a heterodisperse input (R. F. Hounam and R. J. Sherwood xe2x80x9cThe cascade Centripeter: a device for determining the concentration and size distribution of aerosolsxe2x80x9d American Industrial Hygiene Assoc. J, 26, pp 122-131, 1965). The relatively monodisperse particles were sampled from the minor flow stream in the second virtual impactor. Another example of using two virtual impactors for size fractionated aerosol samples is given by Chen et al (B. T. Chen et al xe2x80x9cUse of two virtual impactors in series as an aerosol generatorxe2x80x9d J. Aerosol Science, 19, pp 137-146, 1988). Marple et al describe an air sampler having two stages of virtual impaction in U.S. Pat. No. 5,040,424 and European patent EU 0 352 126 A2. The two stages of virtual impaction are used for separating particles of different size. Another multi-stage virtual impactor for separating particles into different size ranges was described by Brenizer et al in internationalpatent WO 99/26464. The pressure drop across a multi-stage virtual impactor increases with each additional stage.
Yule described the use of a multi-jet, multi stage virtual impactor using round jets for concentrating particles in U.S. Pat. No. 4,132,894. Burton et al described multi-stage virtual impactors with slit jets to increase the aerosol concentration of a given size in U.S. Pat. Nos. 5,425,802 and 5,788,741 and in international patent WO 94/25141. Sioutas et al further described the use of multi-stage slit impactors for concentrating particles (C. Sioutas et al xe2x80x9cDevelopment and evaluation of a prototype ambient particle concentrator for inhalation exposure studies, Inhalation Toxicology, 7, 633-644, 1995; C. Sioutas et al xe2x80x9cA technique to expose animal to concentrated fine ambient aerosolsxe2x80x9d. Environmental Health Perspectives, 103, 172-177, 1995; C. Sioutas et al xe2x80x9cFine particle concentrators for inhalation exposures- effect of particle size and comoposition, J Aerosol Sci, 28, 1057-1071, 1997).
The present invention provides a system for separating a fluid flow into a first particle flow component containing first size particles essentially greater than a selected size and a second particle flow component containing second sized particles essentially smaller than the selected size. In one embodiment of the invention the fluid flow is directed into a number of slit nozzles. The slit nozzles are positioned so that the fluid flow is directed into a gap between the nozzles and (a) a number of receiving chamber inlets leading into receiving chambers and (b) a number of exhaust chambers. The receiving chambers are in alignment with the slit nozzles. The exhaust chambers are located offset from the aligned slit nozzles and receiving chambers. The size of the slit nozzles, the gaps, and the receiving chamber inlets are selected so that the fluid flow will be separated into (a) the first particle flow component with said larger particles which is directed into the receiving chambers, and (b) the second particle flow component with the smaller particles which is directed into the exhaust chambers.
In a preferred embodiment of the invention, a virtual impactor system is provided for dividing a particle containing gas flow into (a) a small flow component with a small portion of the gas flow that carries particles essentially greater than a predetermined size and (b) a large flow component with a large portion of the gas flow that carries particles essentially less than the predetermined size. The gas can be either air or other gases. The virtual impactor system can also utilize fluids other than gas, for example liquids.
The virtual impactor system includes multiple nozzles for accelerating and channeling the fluid stream and multiple receivers positioned downstream from the nozzles. The nozzles are located so that each receiver has a substantially common axis with a corresponding nozzle. The receivers are separated from the nozzles by gaps. The virtual impactor system also includes multiple exhaust chambers in fluid communication with the gaps between the nozzles and the receivers. The multiple exhaust chambers are sandwiched alternately between the multiple receivers. The nozzles have an elongated outlet, with the nozzle outlet positioned to direct the accelerated fluid flow into the receivers in an approximate straight line.
In one embodiment of the invention the receivers have an elongated inlet and outlet, with the outlet of the receivers directed downstream. The receivers will contain the large particle flow component. The fluid exhaust chambers each comprise a chamber having an elongated inlet in fluid communication with the neighboring gaps between the nozzle outlet and the receiving inlet and are located approximately parallel to the receivers. The fluid exhaust chambers will contain the small particle flow component. The fluid exhaust chambers have an outlet at one or both of the narrow sides of the exhaust chamber, the narrow sides being at approximately right angles to the inlet of the exhaust chamber. The fluid exhaust chambers have a sealed side that is opposite the inlet, thereby directing the small particle flow component from the inlet to the side exhaust.
A fluid flow device is used to drive the fluid through the multiple nozzles and out from the outlets of the receivers and the outlets of the multiple exhaust chambers. The fluid flow device may consist of a first vacuum fluidly connected to the outlets of the receivers and a second vacuum fluidly connected to the outlets of the multiple exhaust chambers.
Accordingly, it is an aspect of the present invention to overcome the deficiencies in the prior art and provide a virtual impactor system that has a simple geometry, high fluid flow, and low pressure drop.
Another aspect of the present invention is to provide a system for separating and concentrating larger particles from a fluid stream using a virtual impactor that has a simple geometry, high fluid flow, and low pressure drop.
Another aspect of this invention is a method for separating and concentrating large particles from a fluid stream using the virtual impactor system described herein.
An additional aspect of the present invention is to provide a micro-machined virtual impactor system having high fluid flow and low pressure drop.
An additional aspect of the present invention is to provide a virtual impactor system that is easier to construct using a micro-machining technology.
An additional aspect of this invention is a micro-machined virtual impactor apparatus using the virtual impactor system design described herein.
Additional aspects, advantages, and features of the invention are set forth in part in the description which follows. Various aspects, advantages, and features of the invention will become apparent to those skilled in the art upon examination of the description and by practice of the invention.